What is a good way to temporarilly dye hair for a week?

go to the salon and ask for a temporary color. The molecules in the color don't penetrate into the hair and because they're on the outside of the hair they will only last a certain number of shampoos. Do not use a semi-perminent color because 1/2 of the molecules go into the hair and will stay.
I speak from experience. How can you do this easily and cheaply?